The invention relates to switching of an AC machine between power from different sources, such as power from supply mains of a power utility and power from a machine drive, such as a variable frequency AC motor drive. More specifically, the invention relates to a method and apparatus for deriving a line transfer reference signal to determine characteristics, such as the phase and magnitude, of the utility or other power supply for synchronizing the phase and magnitude of the two power supplies without the need for an external sensor.
In many applications, it is desirable to switch an AC machine receiving power directly from supply mains to receive power from a drive, or vice versa. The phrase xe2x80x9csupply mainsxe2x80x9d, as used herein, refers to any source of AC electric power having a frequency that is determined by the power utility. The term xe2x80x9cdrivexe2x80x9d as used herein refers to any source of AC electric power having a frequency that is generated by a mechanism other than the power utility, such as a three phase AC variable frequency motor drive, a smart motor starter, or the like. The phrase xe2x80x9cAC machinexe2x80x9d, as used herein refers to any AC rotating machine, such as an inductance motor, a synchronous motor, a generator, a dynamo or the like.
For example, in gas turbine static starters, a variable frequency motor drive is often used to bring a generator motor up to a self-sustaining speed at which the generator motor can subsequently be switched to supply mains. In other applications, such as variable speed fans and pumps, it is desirable to achieve variable speed with a variable frequency drive and subsequently switch the motor to supply mains for sustained operation at a constant high speed. Also, in the event of failure of the motor drive, it is desirable to switch the motor to the supply mains to continue operation. Similarly, it is often desirable to switch a motor from supply mains back to a motor drive for slowing the motor down or otherwise varying the speed or torque of the motor.
It is well known to synchronize the phase and magnitude of signals from supply mains and a drive to accomplish a xe2x80x9cbumplessxe2x80x9d transfer. However, known techniques for synchronization require an external sensor for detecting zero crossings or magnitude and phase of the utility. Of course, the use of an external sensor increases the cost and complexity of the system. Also, the external sensor may itself contribute to phase differences further complicating the system. U.S. Pat. No. 5,587,474 discloses a control system in which the phase difference of an input PLL and an output PLL of an inverter is driven to zero to synchronize output of the inverter to output of supply mains. However, this system is extremely complex. Also, there are applications in which synchronization should be accomplished between a motor drive output and sources other than the supply mains powering the motor drive. The system disclosed in U.S. Pat. No. 5,587,474 is not readily adapted to such synchronization. Switching of a motor from a motor drive to supply mains is referred to xe2x80x9ctransferxe2x80x9d herein and switching of a motor from supply mains to a motor drive is referred to as a xe2x80x9ccapturexe2x80x9d herein.
A first aspect of the invention is directed to a method for deriving characteristics of a signal from a utility in an AC machine drive system. The method comprises sending an input signal to a drive while the drive is idle, and sending a signal from a utility to a an AC machine coupled to an output of the drive. Additionally, characteristic of a signal is detected at the output of the drive while the drive is idle and characteristic of the signal is detected from the utility. Next, the characteristic of the signal at the output of the drive is compared to the characteristic of the signal from the utility to determine a characteristic adjustment. Finally, the characteristic adjustment is applied to the characteristic of the signal from the utility to derive a characteristic of a signal at the AC machine due to the utility.
A second aspect of the invention is directed to a drive system capable of switching an AC machine between power from a drive and power from a utility. The drive system includes a utility, a drive, an AC machine, a utility contactor coupled between the utility and the AC machine to selectively couple an output of the utility to the AC machine, a drive contactor coupled between the drive and the AC machine to selectively couple an output of the drive to the AC machine, and a controller. The controller is operative to detect a characteristic of a signal at the output of the drive due to power from the utility when the drive is idle and the utility contactor and the drive contactor are closed. The controller is also operative to detect a characteristic of the utility feed to the AC machine and compare the characteristic of the signal at the output of the drive to the characteristic of the signal from the utility to determine a characteristic adjustment.
A third aspect of the invention is directed to an apparatus for deriving characteristics of a signal from a utility in an AC machine drive system. The system comprises structure for sending a signal from a utility to an AC machine coupled to an output of the drive, structure for detecting the characteristic of a signal at the output of the drive due to the power from the utility, and a means for detecting a characteristic of the signal from the utility. The system also includes structure for comparing the characteristic of the signal at the output of the drive to the characteristic of the signal from the utility to determine a characteristic adjustment, and structure for applying the characteristic adjustment to the characteristic of the signal from the utility to derive a characteristic of a signal at the AC machine due to the utility.